<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <button id="btn">按钮</button>
  </div>
  <script>
    btn.addEventListener('click', function() {
      console.log("btn被点击")
    }, false);
    btn.addEventListener('click', function() {
      console.log("btn被点击2")
    }, true);
    app.addEventListener('click', function(w) {
      w.preaf
      console.log("app被点击");
    }, true);
    // 按钮点击后，app先触发，btn后触发。
  </script>
</body>
</html>